Chapter 9 – A View of Earth’s Past
We are used to time divisions such as days, months, and years. With geologic time, we use eons, eras, periods, and
epochs.
The largest unit of geologic time is an eon. The four eons are the Hadean, Archean, Proterozoic, and the Phanerozoic.
The first three eons are grouped together as Precambrian Time. While the Precambrian spans four billion years, very
few fossils existed or were preserved, so Precambrian time is not divided further.
The Phanerozoic eon is divided into three eras: the Paleozoic (meaning “ancient life”), the Mesozoic (meaning “middle
life”), and the Cenozoic (meaning “recent life”). Each of these eras is further divided into periods.
Geologic time intervals are divided based on a change in the fossil record and are not equal in time intervals. The fossil
record shows a gradual development of new organisms from preexisting organisms called evolution. There are also
major extinctions that occur at the end of the Paleozoic and Mesozoic eras. These extinctions allow new niches for other
species to move into and flourish. In the Cenozoic era, the periods can be divided into smaller units of time called
epochs.
The pneumonic device for remembering the periods in order, starting with the oldest is: Come Over Some Day, Maybe
Play Poker, Three Jacks Cover Two Queens. The first letter of each of these words gives you the first letter of each of the
periods in order. The Mississippian and Pennsylvanian periods are often grouped together as the Carboniferous period.
During this time there were huge forests in swampy lowlands which became the coal deposits we find today in
southwestern Virginia and West Virginia.
Stromatolites, which are mats of blue-green algae are the most common Precambrian fossil.
During the Paleozoic, trilobites flourished in the warm, shallow oceans. By the end of the Paleozoic they were extinct.
Also, during the Paleozoic, Eurypterids were common. They resemble a scorpion or lobster and were up to nine feet
long. Crinoids, thought to be ancestors of modern sea stars, also flourished in the seas. At the end of the Paleozoic, a
mass extinction occurred, and 90% of marine organisms and 78% of land organisms died. This mass extinction provided
the opportunity for new species to develop.
The Mesozoic is known as the Age of Reptiles. This is the time of the dinosaurs, with pterosaurs in the air, ichthyosaurs
and ammonites in the oceans, and Brontosaurus, Stegosaurus, and Tyrannosaurus rex on land. During the early part of
the Mesozoic, cone-bearing plants called cycads dominated. But by the end of this era, a major change in the plant
world occurred with angiosperms, flowering plants, developing. Just as in the Paleozoic, at the end of the Mesozoic, an
enormous mass extinction occurred wiping out the dinosaurs and many other species. The hypothesis is that a large
meteorite hit the Earth near the present-day Gulf of Mexico, throwing enough material into the atmosphere to block
out solar radiation and decrease global temperatures. This event, combined with already stressed ecosystems due to
volcanism, sea level changes, and active plate tectonics, led to mass extinctions.
The Cenozoic is when mammals flourished. The four major ice ages occurred in this era. Mammals were more adapted
to the cold temperatures than reptiles because: they had hair covering their bodies, a layer of fat inside their bodies
acting as insulation, and were warm-blooded, meaning they did not have to absorb heat from their surroundings. The
tarsier is a modern survivor of a group of primates from the early Cenozoic. There is enough fossil evidence in the
Cenozoic to divide periods into smaller units of time called epochs. The current epoch is called the Holocene. During this
epoch, the last major ice age ended about 11,500 years ago, sea level rose about 140 meters as the ice melted, the
Great Lakes formed, and modern humans (Homo sapiens) developed.